Sheet jointing and form of a roche moutonnée on the N
side of Loch Avon after Sugden and John (1976)

An explanation of the origin of the term
roche moutonnée
from the excellent interpretative site at
Dulnain Bridge

Sugden et al. (1992) have provided a useful
model of how spurs in the Dee valley have been modified beneath thick ice to
form large roches moutonnées.

Stac an Fharaidh on the plateau above Loch Avon. The preglacial spur
has been plucked on the left (down-ice) face to give cliffs.
Weathering pits >10 cm deep on the upper surface of the roche
moutonnée indicate formation before the last interglacial

Roches moutonnées

Definition: A roche moutonnée is a rock hill shaped by the passage of ice to give a smooth
up-ice side and a rough, plucked and cliff-girt surface on the down-ice side. The
upstream surface is often marked with striations.
Whalebacks are sister forms.

Cnapan a’ Mheirlich, Glen Avon

Roches
moutonnées occur widely in the Cairngorms, both on the granite and on the
surrounding metamorphic country rocks. Like other landforms of glacial erosion,
roches moutonnées are usually absent from the Cairngorm plateau, although
good examples occur at the head of Glen Avon. The roches moutonnées vary
greatly in size. Ice-moulded hills in Strathspey, such as
Ord Bàn, and in Glen
Dee, such as Craig Leek, have a relative relief of 200-300 m. In contrast, the
granite roches moutonnées in the valley of the Garbh Uisge Mhor, on the
slopes of Ben Macdui, are only a few metres in height.

The
roches moutonnées of the Cairngorms provide insights into a number of
problems of the glacial history of the area:

Patterns
of ice flow: roche moutonnées are aligned roughly parallel to ice flow.
The mega-forms of Strathspey and the Dee valley mark the passage of thick ice
along these corridors, whilst the roches moutonnées above Glen Avon reflect
ice flow towards the northeast.

Several
factors complicate this basic relationship between orientation and patterns of
former ice flow.
Rock structure can favour the development of ice-moulded
hills that do not parallel ice flow (Gordon, 1981), although in the Cairngorms the
orientation of roche moutonnées generally conforms to that other indicators
of ice flow. More significantly, large roche moutonnées have been shaped
during multiple phases of glaciation. The direction of ice flow may have been
similar in each phase but, in some places, it was not. The spurs of the
southern Cairngorms, such as Carn Crom, show a dominance of lee-side plucking
on eastern faces but also signs of plucking on southern faces. This duality
may reflect two directions of ice flow; one from the Lairig Ghru and the
other, more restricted in time or erosional capacity, from Glen Derry. Similarly, on Cnapan
a' Mheirlich (above) there is a scatter of metamorphic erratics which reflects
a late phase of ice flow that is not related to the many phases of
northeasterly flow that shaped the hill.

Depths
of glacial erosion: at a few locations, small roches moutonnées occur
downslope of tors and represent the glacial modification of pre-existing
forms. One of the best examples occurs on the northern spur of Beinn
Mheadhoin.

The evolution of large roches moutonnées
can be traced by using the curved sheet joints of granite spurs as indicators
of the preglacial form. Where remnants of tors occur upslope, the bites taken
out from the spur by leeside plucking represent the sum of Quaternary glacial
erosion (Sugden et al., 1992).

Former
glacier basal thermal regime: both the abraded stoss side and the plucked
lee side of roches moutonnées require the former presence of sliding ice and
meltwater. The presence of roches moutonnées means therefore that the ice
masses that progressively shaped these bedrock hills were warm based, with ice
at the glacier bed above its pressure melting point. The valley-floor roches moutonnées
reflect the passage of thick, fast-moving and sliding ice streams. The
high-level roches moutonnées above Loch Avon indicate that during some phases
of the Quaternary warm-based ice crossed the edges of the surrounding plateau.

Age of
features: low-level roches moutonnées show only shallow weathering pits,
indicating significant erosion by the last ice sheet. Some high-level
ice-moulded rock surfaces have deep weathering pits indicating formation before
the last ice sheet.